I just booked our air to SXM for this fall, and booking the legs separately on AA instead of booking round trip saved $75/ticket. I've never encountered this before on a single airline. Saved additionally because they still allow a 24 hour hold on tickets, and the tickets went up overnight.

I've been searching for weeks for the perfect flights, i.e. no 6 hour layover in MIA, and finally settled for 2nd best each way. Except, the second best going is the best schedule, but a day earlier. We'll just have to live with an extra day on the island.

BTW, for you left coasters, the fare was $438 rt from SFO, so the planets have aligned for us this trip.